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Create A High Input Impedance, Rail-To-Rail Measurement System

Mark Thoren | Jan 18, 2007

Two very desirable features for a
precision measurement system based
on an analog-to-digital converter (ADC)
are high input impedance and a wide
input range, ideally including or extending slightly beyond the power-supply
rails. The circuit described here does
just that. Its very high input impedance
is complemented by an input range that
extends 300 mV beyond the supply rails.

The example circuit uses a thermocouple and a resistance temperature
detector (RTD) connected to an LTC2449
high-performance delta-sigma ADC (Fig. 1). Thermocouple outputs produce very
small changes (tens of microvolts per
degree C), and the output will be negative if the thermocouple junction is colder than the "cold junction" connection
from the thermocouple to the copper
traces on the pc board.

The RTD is measured by comparing
the voltage across the RTD to the voltage
across the reference resistor, RREF. This
provides a very precise resistance comparison, and it doesn't require a precise
current source. Grounding the sensors
as shown is a good first line of defense
for reducing noise pickup. However, the
ADC must accommodate input signals
that are very close to, or slightly outside,
the supply rails. The LTC2449 handles
these signals very well.

The analog inputs to the ADC are routed to the device's MUXOUT pins, and an
external buffer isolates these signals
from the switched-capacitor ADC inputs.
The external buffer, an LTC6241 precision CMOS dual op amp, provides a high
impedance through the multiplexer and
back to the analog inputs. This has a distinct advantage over integrated buffers
because the analog inputs are truly railto-rail—and even slightly beyond—with
appropriate buffer supply voltages.

The LTC6241 has a rail-to-rail output
stage and an input common-mode
range from the negative supply to 1.5 V lower than the positive supply.
Because no rail-to-rail amplifier can
actually pull its outputs to the rails, an
LT3472 boost/inverting regulator is
used to create a 1.25/7-V supply for
the op amp from the 5-V LTC2449 supply (Fig. 2). The regulator can provide
enough current for several amplifiers
and other circuitry that really needs to
swing to the rails.

In addition, the LT3472's 1.1-MHz
switching frequency is close to the middle of the LTC2449 digital filter stopband. The center of the stopband is 900
kHz when using the internal conversion
clock and is independent of the selected
speed mode. The amplifier's 0.01- µF capacitive load and compensation network provide the ADC with a "charge
reservoir" to average the ADC's sampling
current while the 2.5-k feedback resistor maintains dc accuracy.